Character interludes are a part of the Halo 3 campaign, in which either Cortana or the Gravemind telepathically address the Master Chief and often muse introspectively. During these interludes, the player's movements will slow drastically, as will their ability to turn and face another direction. A screen effect will alter the display of the game.
During a Cortana interlude, the screen turns blue, and Cortana's image flashes in and out of focus on the screen. Her voice will contain a slight echo and sometimes a deeply-pitched distortion effect, and her image will erratically "jump" around the screen. During a Cortana moment there will also be a noise sounding vaguely similar to the noise heard if one places a seashell in their ear.
During a Gravemind interlude, the player's view become murky green, and tentacle-like imagery can be seen at the edge of the screen. The screen will shake in unison with the Gravemind's voice, shaking far more violently if Gravemind is screaming or yelling in anger at John-117.
The Cortana interludes depict her descent into rampancy, while the Gravemind moments are used to convey the point of view of the Flood. Checkpoints are usually attained after these moments. The player is almost never in any danger of damage while the interludes occur, which is fortunate, as the player cannot move quickly. It is possible in co-op mode for one player to trigger the interlude while the other player is behind fighting enemies that the first one skipped. During any of these moments, the player is completely immune to damage and will not be attacked even by nearby enemies.
The Halo Evolutions story Human Weakness provides an explanation for the Cortana interludes. It is revealed that the Gravemind had successfully hijacked Cortana's system, allowing him to speak with her voice. This was part of his plan to persuade Cortana and John-117 that Cortana was descending into rampancy. It is, however, not explained how the Gravemind was able to connect to John-117 all the way from High Charity.
Cortana battled the Gravemind, trying to hold onto the data about the Portal and the Ark. She ends up creating multiple copies of herself to distract the Gravemind and twice they apparently chime in during her conversations with the Gravemind. One adds the "I am a monument to all your sins" part of one of the interludes while the rest is Cortana defying Gravemind to buy the Master Chief time. Cortana places the message onto a Flood-controlled cruiser by composing it with the help of a copy and then transmitting it to that ship through Gravemind's link to it, with part of it being coded to seek out and upload itself to the nearest data device. Gravemind then reveals that he knew and helped out as he wanted to lure all of his enemies to one place, but had no idea what the message actually said.
These moments happen during a cinematic and Cortana's face is not seen except for the last one listed.
This and the last Arrival interlude are the only times any NPCs notice them.